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GPS glitched across the US by as much as 33 feet

sciencealert.com

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Re: GPS glitched across the US by as much as 33 feet

#71

Post by Quibblingeek »

ashleyn wrote: post #60
Quibblingeek wrote: post #40

Earlier quoted context omitted.

Maybe not intentionally (at least by the U.S. Government, but the consequences are far greater now. Now that GPS has subsumed a large part of air navigation as more VOR and ILS approaches at non-towered airports are being left to fall into disrepair, it forces more and more aircraft operating in instrument conditions to fall back to a navigation system that may not operate to the tolerances that you need to shoot an…

One of the reasons SA was discontinued was that if you knew the coordinates of a base station, you could correct for the error. Are airports not doing this already?

Actually, several airports are. So you have normal WAAS and it has a ground segment. Most (new) IFR certified GPS receivers installed in airplanes today are WAAS capable.

To that, you can augment something called GBAS (we used to call it Local Area Augmentation System but switched over to the ICAO name for it). It’s basically a ground station that sits at an airport and corrects for GPS errors where you need it most (airports definitely qualify here and that’s where you need the best GPS performance).

Ultimately, the FAA is going to go 100% GPS and it’ll fall back to the minimum number of land-based sources for redundancy. It’s about cost. The issues with GPS may cause them to push the timeline back and develop plans to mitigate the issue, but they’re too far down this path to stop now.

Re: GPS glitched across the US by as much as 33 feet

#72

Post by rafram »

avazhi wrote: post #54
boomboomsubban wrote: post #46

Earlier quoted context omitted.

The trackers are part of CCSO’s pretrial house arrest program, so presumably a number of them are attached to people who did no crimes.

> so presumably a number of them are attached to people who did no crimes. You and I have very divergent views of the type of person likely to be charged or prosecuted for a crime or indeed to participate in a diversion program that involves GPS trackers, my friend.

They are innocent until they’re proven guilty.

Re: GPS glitched across the US by as much as 33 feet

#73

Post by rotringlvr »

delta_p_delta_x wrote: post #8

*10 metres

Feet are actually great units for RF because a nanosecond is very close to a foot. Since feet has many prime factors and/or the foot-meter conversion is also easy, you can quickly get a wavelength of, say a 2GHz (6 inches), and get it in cm (15cm).

Of course, ideally we'd ditch the silly metric system and use natural units. Then these 30 feet are a more comfortable 50M/eV. But those SI guys are really conservative and attached to confusing unit system based on fractions of the diameter of an arbitrary planet.

Re: GPS glitched across the US by as much as 33 feet

#74

Post by [deleted] »

avazhi wrote: post #54
boomboomsubban wrote: post #46

Earlier quoted context omitted.

The trackers are part of CCSO’s pretrial house arrest program, so presumably a number of them are attached to people who did no crimes.

> so presumably a number of them are attached to people who did no crimes. You and I have very divergent views of the type of person likely to be charged or prosecuted for a crime or indeed to participate in a diversion program that involves GPS trackers, my friend.

[deleted]

Re: GPS glitched across the US by as much as 33 feet

#76

Post by [deleted] »

rafram wrote: post #72
avazhi wrote: post #54

Earlier quoted context omitted.

> so presumably a number of them are attached to people who did no crimes. You and I have very divergent views of the type of person likely to be charged or prosecuted for a crime or indeed to participate in a diversion program that involves GPS trackers, my friend.

They are innocent until they’re proven guilty.

[deleted]

Re: GPS glitched across the US by as much as 33 feet

#77

Post by Aurornis »

chaps wrote: post #70
Aurornis wrote: post #68

Earlier quoted context omitted.

Geofence monitors have buffer zones because a small GPS on someone’s ankle inside a house, potentially a high rise, isn’t going to be that precise. 33 feet is well within the allowances IIRC. Varies by state.

That might be true for a place in the middle of nowhere with no features, the same device model managed by the same company and the same call center person and same sheriff and judges and prosecutor. But with GPS in an urban environment where many homes are over 100 years built from many different materials, 33 feet is a lot.

Urban environments have larger buffer areas due to multipath interference.

The story you linked has examples about false alerts triggering in a different state.

Do you have any actual examples of 33 feet getting someone arrested? I’ve worked enough with GPS that I could tell you that any system with thresholds that low would result in everyone getting false triggers

Re: GPS glitched across the US by as much as 33 feet

#78

Post by chaps »

Aurornis wrote: post #77
chaps wrote: post #70

Earlier quoted context omitted.

That might be true for a place in the middle of nowhere with no features, the same device model managed by the same company and the same call center person and same sheriff and judges and prosecutor. But with GPS in an urban environment where many homes are over 100 years built from many different materials, 33 feet is a lot.

Urban environments have larger buffer areas due to multipath interference. The story you linked has examples about false alerts triggering in a different state. Do you have any actual examples of 33 feet getting someone arrested? I’ve worked enough with GPS that I could tell you that any system with thresholds that low would result in everyone getting false triggers

Yes, everyone on the devices received false alarms. It's why there was a call center that managed the false alarm flows. I highly suggest you read the articles I've shared.

I fought tooth and nail to get access to the data showing how often devices reported "invalid" status (where the device's specs said it was something like, within 150 feet of inaccuracy it reported "invalid"). They initially told me they couldn't get me that data because it'd crash their database running that query. Basically threatened to sue them and over the next 6mo or so I got two 100GB MSSQL dumps. Hundreds of millions of rows of devices reporting back that their GPS signal was invalid. It would take a lot of time to make it sharable because they included the actual GPS location infos.

Re: GPS glitched across the US by as much as 33 feet

#79

Post by entrope »

Quibblingeek wrote: post #71
ashleyn wrote: post #60

Earlier quoted context omitted.

One of the reasons SA was discontinued was that if you knew the coordinates of a base station, you could correct for the error. Are airports not doing this already?

Actually, several airports are. So you have normal WAAS and it has a ground segment. Most (new) IFR certified GPS receivers installed in airplanes today are WAAS capable. To that, you can augment something called GBAS (we used to call it Local Area Augmentation System but switched over to the ICAO name for it). It’s basically a ground station that sits at an airport and corrects for GPS errors where you need it most…

To meet its integrity budget, a GBAS needs at least triple redundancy, not just a single station.

If one has a lot of patience, one can read the details of how it works at https://elibrary.icao.int/product/299828 (along with SBAS, the ICAO name for what WAAS does; the constellations they can augment; GRAS, a weird hybrid between GBAS and SBAS that apparently went nowhere; ILS; VOR; DME; and VHF marker beacons). Annex 10 Vol I probably doesn't say explicitly that triple redundancy is required, because if you had a single receiver with an MTBF of something like 10 billion hours then you could use just that one.

This particular 30-foot error should go away almost entirely for future receivers that implement DFMC SBAS or ARAIM.

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